Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Real Estate Acquired Through Foreclosure Under Forward Purchase Agreements [Line Items]
Statutory income tax rate	35.00%
Increase in valuation allowance	$ 9,700,000	$ 51,800,000
Valuation allowance related to stock-based compensation and exercises prior to the implementation of ASC 515 and 718	35,600,000	35,600,000
Deferred U.S. income taxes	0	422,000	(422,000)
Income taxes have not been provided on a cumulative total	3,900,000
Income tax research credits	2,933,000	6,273,000	591,000
Federal orphan drug credits	17,700,000
Federal orphan drug credits, expiration year	2026
Unrecognized tax benefits, period may increase or decrease due to tax examination	Next twelve months
Federal [Member]
Real Estate Acquired Through Foreclosure Under Forward Purchase Agreements [Line Items]
Net operating loss carryforward for income tax	1,131,800,000
Net operating loss carryforward, expiration year	2018
Income tax research credits	40,000,000
Income tax research credits, expiration year	2019
State [Member]
Real Estate Acquired Through Foreclosure Under Forward Purchase Agreements [Line Items]
Net operating loss carryforward for income tax	709,200,000
Net operating loss carryforward, expiration year	2015
Amounts included in state net operating losses not MLTN	124,600,000
Income tax research credits	$ 22,600,000